1. Technical Field
The present invention relates to an antenna structure provided for a radio communication apparatus, such as a cellular phone, and a radio communication apparatus including such an antenna structure.
2. Background Art
FIG. 9a schematically shows an example of an antenna structure (see, for example, Patent Document 1). An antenna structure 40 includes a bar-shaped radiation conductor 41, a coaxial cable 42, and a feeder line 43. The bar-shaped radiation conductor 41 functions as an antenna in accordance with a resonant operation and has a line length X (X=λ/4), which is approximately one-quarter the wavelength λ of a radio wave at a resonant frequency set in a frequency band defined in advance for radio communication. The coaxial cable 42 includes an internal conductor (core wire) 42a and an external conductor 42b that is arranged circumferentially around the internal conductor 42a with a gap therebetween. A rear end of the coaxial cable 42 (the left end in FIG. 9a) serves as a connection end, and one end of the feeder line 43 is connected to a connection end of the internal conductor 42a of the coaxial cable 42. The other end of the feeder line 43 is electrically connected to a radio communication circuit 44 provided in a radio communication apparatus. In addition, a connection end of the external conductor 42b of the coaxial cable 42 is electrically connected via a lead D to one end (a rear end) of the radiation conductor 41.
The coaxial cable 42 functions as an impedance circuit for achieving impedance matching between the radiation conductor 41 and the radio communication circuit 44. The coaxial cable 42 functions as an inductance as represented by an equivalent circuit shown in FIG. 9b or functions as a capacitor as represented by an equivalent circuit shown in FIG. 9c, by appropriately setting the state of connection between a leading end of the internal conductor 42a and a leading end of the external conductor 42b (that is, whether or not the leading ends are connected to each other) and setting the line length of the coaxial cable 42. Thus, the state of the connection between the leading ends of the internal conductor 42a and the external conductor 42b of the coaxial cable 42, the line length of the coaxial cable 42, and other factors known to those skilled in the art, are set in an appropriate manner such that impedance matching between the radiation conductor 41 and the radio communication circuit 44 can be achieved.
The antenna structure 40 is configured as described above. For example, when a transmission signal is transmitted from the radio communication circuit 44 via the feeder line 43 and the coaxial cable 42 to the radiation conductor 41, the transmission of the signal causes the radiation conductor 41 to perform a resonant operation and the signal is radio-transmitted. In addition, when a signal arrives at the radiation conductor 41 and the radiation conductor 41 performs a resonant operation and receives the signal, the received signal is transmitted via the coaxial cable 42 and the feeder line 43 to the radio communication circuit 44.
FIG. 10 shows an example of another form of antenna structure (see, for example, Patent Document 2). An antenna structure 45 shown in FIG. 10 is capable of implementing radio communication in two different radio communication frequency bands. The antenna structure 45 includes a line-shaped antenna element 46 and a trap circuit 47. The line-shaped antenna element 46 performs transmission and reception of radio waves in accordance with a resonant operation. One end of the line-shaped antenna 46 (the left end in FIG. 10) serves as a feeding end, and the feeding end is electrically connected to a radio communication circuit 48. In addition, the other end of the line-shaped antenna element 46 (the right end in FIG. 10) serves as an open end. The line-shaped antenna element 46 has the configuration described below, such that the line-shaped antenna element 46 is capable of functioning as an antenna by resonating in two different frequency bands defined in advance for radio communication.
That is, the line-shaped antenna element 46 is caused to perform a resonant operation at a resonant frequency Flow set in the lower frequency band of the two different frequency bands defined in advance for radio communication, and a resonant operation at a resonant frequency Fhi set in the higher frequency band of the two different frequency bands defined in advance for radio communication. To achieve such operation, the trap circuit 47 is provided in the line-shaped antenna element 46. The trap circuit 47 is provided in the line-shaped antenna element 46 at a position where the electrical length Y from the feeding end is the same as one-quarter the wavelength λhi of a radio wave at the resonant frequency Fhi set in the higher frequency band for radio communication. The trap circuit 47 is an LC resonant circuit including a capacitor 49 and an inductor 50. The capacitance of the capacitor 49 and the inductance of the inductor 50 are set such that antiresonance occurs at the resonant frequency Fhi set in the higher frequency band for radio communication. Due to the provision of the trap circuit 47, when the open end is viewed from the feeding end of the line-shaped antenna element 46 at the resonant frequency Fhi set in the higher frequency band for radio communication, in the antenna element 46, an area from the trap circuit 47 to the open end is not electrically visible. Thus, in the case of radio communication in the higher frequency band for radio communication, in the line-shaped antenna element 46, an area from the feeding end to the position where the trap circuit 47 is provided resonates at the resonant frequency Fhi, and thus radio communication is implemented.
In addition, in terms of the resonant frequency Flow set in the lower frequency band for radio communication, the trap circuit 47 functions as a circuit for providing a reactance to the line-shaped antenna element 46. Thus, the line-shaped antenna element 46 is designed such that the electric length (electrical length) from the feeding end to the open end of the line-shaped antenna element 46 is approximately one-quarter the wavelength λlow of a radio wave at the resonant frequency Flow set in the lower frequency band for radio communication while taking into consideration the reactance to be provided. Thus, in the case of radio communication in the lower frequency band for radio communication, the entire line-shaped antenna element 46 resonates at the resonant frequency Flow set in the lower frequency band for radio communication, and thus radio communication is implemented.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-266526    Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-88032
In the configuration of the antenna structure 40 shown in FIG. 9a, for example, in order to connect the radiation conductor 41 to the coaxial cable 42, a process to achieve connection between the radiation conductor 41 and the lead D and connection between the coaxial cable 42 and the lead D, by soldering or the like, is necessary. Thus, a problem occurs in which a manufacturing process becomes more complicated. In addition, there is another problem, in that it is troublesome to carry out assembly processing (positioning) of the radiation conductor 41, the lead D, and the coaxial cable 42 in such a connecting process. As stated above, since it takes much time and effort to produce the antenna structure 40, there is a problem in which the manufacturing cost of the antenna structure 40 increases. Furthermore, since the connection status of the portions connected by soldering cannot be maintained constant all the time, there is another problem in which a variation in the antenna characteristic occurs due to a variation in the connection status of the connected portions.
Regarding the antenna structure 45 shown in FIG. 10, since the trap circuit 47 must be built in the line-shaped antenna element 46, there is a problem in which the manufacturing process becomes more complicated. In addition, there is another problem in which a variation in the antenna characteristic occurs due to a variation in the position in which the trap circuit 47 is built.